This invention relates to the art of induction heating and, more particularly, to method and apparatus for uniformly heating an elongated workpiece having a uniform cross-section over the major portion of its length with at least one enlargement disposed therealong.
The invention is particularly applicable to heating sucker rods which are employed in oil wells and the like and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications and may be used for inductively heating a variety of elongated workpieces having one or more areas of enlarged cross-section disposed therealong.
Sucker rods are utilized in the petroleum industry as a connecting link between a down hole oil well pump and the lifting or pumping device on the surface. Each rod is quite long, normally being on the order of magnitude of 25-30 feet. A major portion of the rod has a uniform cross-section with an enlargement being included adjacent each end thereof. These enlargements variously facilitate interconnecting a plurality of the rods in an end to end relationship with each other. While there are a number of specific or detailed modifications which may be included in the sucker rod configurations of different manufacturers, almost all such rods have the foregoing general conformation and characteristics.
As one of the manufacturing steps, the sucker rods are heated some predetermined temperature and then passed through an electrostatic spray chamber for having a coating of paint or plastic-like material applied thereto. Since the rods need only be surface heated for this purpose, induction type heating finds particular use in this environment. However, because sucker rods do not have uniform cross-sections throughout the entirety of their lengths, it is necessary to in some way compensate for the heating variations which otherwise occur at the enlargements adjacent the rod ends. Unless each rod is heated to a uniform temperature over the entire length thereof, the paint of plastic-like coating subsequently applied will not have a uniform consistency, thickness, etc. thereon, particularly as between the elongated rod portion and the enlargements.
A number of different arrangements to compensate for this difficulty have previously been proposed, developed and attempted. Such solutions include voltage regulation for the inductor, delay on and off timers, various alternative inductor configurations and the like. However, none of these proposed modifications have satisfactorily solved the problem of uniformly heating sucker rods or other elongated workpieces which have at least one enlarged area located at some point therealong.
It has, therefore, been considered desirable to develop an arrangement which facilitates uniform induction heating of sucker rods and other workpieces of the type described. The subject invention is deemed to provide method and apparatus which successfully overcome the foregoing problems. Moreover, the invention is equally applicable to use in tempering, case hardening, through hardening and normalizing of sucker rods or other workpieces.